1. Field of the Invention
The present invention relates to a harmonic reducer, more particularly, relates to a harmonic reducer that is fixed when being applied with a force in one direction, and is free to rotate when being applied with a force in another direction.
2. The Prior Arts
The harmonic reducer is a gear drive which uses flexible components to generate flexible mechanical wave to transmit power and motion. It is also called a “harmonic gear drive” (also known as the harmonic drive).
FIG. 1 shows a conventional harmonic reducer, which includes three components: a circular spline A with an internal gear, a flexspline B with an external gear and a wave generator C. When the harmonic drive functions as a reducer, the conventional type of operation is to set the circular spline A as stationary, the wave generator C as active operating and the flexspline B as the power output.
The flexspline B is a thin-walled gear, which has a greater range of elastic deformation, where the inner diameter is equal or slightly larger than the total outer diameter of the wave generator. The wave generator C is a component, which can elastically deform the flexspline B within a controllable range. The wave generator C includes a central axel, wherein the central axel is installed with a deformable rolling bearing to form a roller. The wave generator C is then assembled inside the flexspline B so the outer wall of the wave generator C and the inner wall of the flexspline B are abutted against each other. The wave generator C is connected to a power source, which is typically a motor. When the wave generator C is installed into the flexspline B, the flexspline is forced to deform from the round shape into an oval shape. The external gear on the two ends of the long axel of the oval are further pushed to be completely engaged with the internal gear of the circular spline A, whereas the external gear near the two ends of the short axel are completely disengaged with the internal gear of the circular spline. The rest of the external gear on other parts of the perimeter is in a transition state between engaged and disengaged. When the wave generator rotates continuously, the deformation of the flexspline also changes continuously, whereas the engaging state between the flexspline B and circular spline A also changes simultaneously. The engagement cycle starts from engaging in, engaged, engaging out, disengaged and then repeats onwards so the flexspline slowly rotates in the opposite direction of the wave generator C relative to the circular spline.
During the driving process, the number of times a point on flexspline B deforms during one revolution of the wave generator is defined as the wave number n. The most widely used drive type is the double wave drive due to its smaller flexspline stress, simpler structure and a greater drive ratio. The pitches of the gears in the flexspline and circular spline of the harmonic gear drive are the same, but the numbers of the gear teeth are not.
When the circular spline is fixed, the wave generator is rotated and the flexspline is driven to rotate. The drive ratio of the harmonic gear drive is equal to: i=−B1/(A1−B1), where A1 and B1 are the number of gear teeth of the circular spline A and the flexspline B, respectively.
Because of the large number of the flexspline gear teeth, the harmonic gear drive has a large drive ratio.
Because of the surface contact between the gear tooth and the high number of simultaneously engaged gear teeth, the harmonic reducer has the advantages of a smaller load per unit area and a higher bearing capacity compared to other drive types during the harmonic drive process. The harmonic reducer also has the benefit of a large drive ratio, where the drive ratio of a single stage harmonic gear drive can be i=70˜500. Furthermore, The harmonic reducer has the advantages of a small size, light weight, high drive efficiency, long service life, high stability, impact free, low noise and high motion accuracy; therefore, the harmonic reducer has a wide range of applications.
However, during the drive process of the reducer, the flexspline needs to endure a larger alternating load which causes the flexspline to be damaged easily. In this case, the required fatigue strength, and the required technique level of process and heat treatment of the flexspline material are higher. The processing technique is also more complex.
In addition, in the conventional harmonic reducer, the distance D between the output shaft and the engaging location of circular spline and flexspline is great, which can cause the sway due to different axel and the increase of heat and noise due to friction.
In order to solve the previous mentioned problems, Taiwan Patent application No. 101210934 filed by the present applicant disclosed a harmonic reducer, which has the wave generator thereof assembled with the flexspline and a power source, so the flexspline deforms and engages with the circular spline partially. The flexspline is fixed while the circular spline is connected to an output shaft. When the power source drives the wave generator to rotate, the flexspline also drives the circular spline and the output shaft to rotate simultaneously.
However, the flexspline of the harmonic reducer described above is mounted directly to the casing of the reducer or other components, so uneven internal stress easily occurs when the flexspline deforms during the driving process. In addition, the amount of deformation in the flexspline is also limited so the adjustment of the tooth difference between the circular spline and the flexspline is not allowed; therefore the need of a greater output range cannot be satisfied through various reduction ratios.
Furthermore, since the flexspline of the conventional harmonic reducer or the reducer disclosed by Taiwan Patent application No. 101210934 are usually disposed fixedly, the output shaft of the circular spline is limited by the resistance thereof, and thereby restricting the use of the harmonic reducer.